The CD4 molecule is a surface glycoprotein to T lymphocytes which not only serves as a recognition factor in the immune response but also is the receptor for the human immunodeficiency virus (HIV). We propose to design and express a soluble version of this molecule and solve the three-dimensional structure by X-ray crystallography. We will first introduce a full-length CD4 cDNA clone into eukaryotic cells and engineer the soluble form by eliminating the membrane anchor at the C-terminus. The carbohydrate attachment sites will also be eliminated in order to produce a protein which, if still soluble, may be more readily crystallized. Both these forms will be tested for activity in binding HIV. Characterization of the soluble CD4 will be done to determine the amino terminus of the mature receptor, disulfide bond assignments, carbohydrate sites and other post-translational modifications. The soluble CD4 will be crystallized and its structure solved by X-ray crystallography. Using the structure of the viral protein which attaches to CD4 (gp120), we will then examine the binding of these two molecules by model studies. Small gp120 peptides implicated in HIV attachment to CD4 will be bound to CD4 in the crystal and these structures solved. Attempts will also be made to co-crystallize whole gp120 and CD4. From these studies we hope to determine the full molecular details of the binding of gp120 to CD4 which would then allow the design of drugs to block this interaction and defeat HIV infection of human T cells.